Gearing for a Very High Speed

[John]

Originally Posted by JorgeReyes

I guess the question I have then is if it is better to automate shifting to shift into low gear when we are drawing too much current and high when we are not, or have the driver shift manually.

This thread may be of interest to you, then.

[notmattlythgoe]

Originally Posted by JorgeReyes

See but the problem I see with this set up is when you want to do something precise like lining up to shoot. If you try to do this on high gear it will be harder to do this. What I was thinking was that it shifts down when it draws to much current to automatically shift in pushing matches like you said.

But also have a manual override option to shift when necessary like the situation I mentioned.

We will also have a manual way to shift down. But our driver has never had an issue lining up to shoot in high gear. Both this season and last our driver was in high gear in all situations except pushing matches.

[Pratik Kunapuli]

Originally Posted by John

~4 fps seems really slow, even to me though. What shifter did you guys use that has a 4.25:1 spread?

Originally Posted by waialua359

You guys spent most of the time driving 4 fps?

Originally Posted by notmattlythgoe

This is the same reaction I had. This can't be the whole story. .

Being in high gear is different from trying to drive at top speed. While we may have been in high gear, I wasn't trying to go very fast a lot of the time, only for the full field dashes. We used low gear when there were defenders near us, we needed to push (partners and opponents), and when we were lining up shots. We had a custom 2 speed gearbox.

[Ether]

Originally Posted by John

if I have time I might add the math to back it up

Here's some quick rough calculations using some simple numbers to illustrate a point.

Say you have a CIM with 10.4 volts applied and loaded with 206.1 oz*in of torque. It will be spinning at 1416 RPM, drawing 80.5 amps, generating 215.8 mechanical output watts, and running at 25.8% efficiency.

Now add a 3x speed reduction gearbox. For sake of simplicity for this simple calculation, ignore gearbox losses. To produce the same output speed and torque as above, the CIM needs to produce 68.7 oz*in torque at 4248 RPM. To generate that torque at that RPM, the CIM will need 12 volts applied. It will draw 28.8 amps and be operating at 62.5% efficiency.

That's why you want to operate in low gear when possible appropriate.

Motor Calculator  build MCALC_2014d 2/3/2014 1255pm


Select motor:
1)CIM     6)FP0673   d)FP2719  f)FP9012     g)FP9015    h)FP9013 j)MiniCIM
a)am-0912 b)am-0914  7)DensoL  8)DensoR     c)Denso0160 k)BAG    u)UserDefined
2)RS395   3)RS540    4)RS550   e)RS775-12   5)RS775-18  m)RS555  n)am-0915
r)am-2193 s)am-2235  t)RS390   v)RS545      p)am-2161&2194
w)VEX2177hi  z)VEX2177std  1


CIM FR801 001, AM802 001A @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    5310   100.0     2.7         0.0        32.4     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   343.4   2.425       0     0.0   133.0         0.0      1596.0     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   171.7   1.212    2655    50.0    67.8       337.1       477.1    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    42.8   0.302    4648    87.5    18.9       147.1        80.2    64.7

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   b)rpm&amps  8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin  m)main menu   x)exit a


Enter motor rpm & ozin, and the program will calculate the motor voltage
required to produce the specified ozin at the specified rpm.

Enter rpm and ozin, separated by a space: 1416 206.1

CIM FR801 001, AM802 001A @ 10.40 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    4603   100.0     2.3         0.0        24.3     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   297.7   2.102       0     0.0   115.3         0.0      1199.3     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   148.8   1.051    2301    50.0    58.8       253.3       358.5    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    37.1   0.262    4029    87.5    16.4       110.6        60.3    64.7

CIM FR801 001, AM802 001A @ 10.40 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   206.1   1.455    1416    30.8    80.5       215.8       622.0    25.8

Select input:
1)oz-in  7)watts_in    3)rpm    5)amps   b)rpm&amps  8)eff%
2)Nm     6)watts_out   4)rpm%   9)volts  a)rpm&ozin  m)main menu   x)exit a


Enter motor rpm & ozin, and the program will calculate the motor voltage
required to produce the specified ozin at the specified rpm.

Enter rpm and ozin, separated by a space: 4248 68.7

CIM FR801 001, AM802 001A @ 12.00 volts:

@ free (no load):
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
     0.0   0.000    5310   100.0     2.7         0.0        32.4     0.0

@ stall:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   343.4   2.425       0     0.0   133.0         0.0      1596.2     0.0

@ max power:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
   171.7   1.213    2655    50.0    67.9       337.1       477.2    41.4

@ max efficiency:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    42.8   0.302    4648    87.5    18.9       147.2        80.2    64.7

CIM FR801 001, AM802 001A @ 12.00 volts:
   oz-in      Nm     rpm    rpm%    amps   watts out  watts heat    eff%
    68.7   0.485    4248    80.0    28.8       215.8       129.4    62.5

[asid61]

I see nothing wrong with gearing for a very high speed, provided you provide some kind of current detection for autoshifting or speed limiting in emergencies.

[Aren Siekmeier]

Originally Posted by Ether

Lots of good math showing considerable differences in current draw

This is really good to see, and demonstrates the importance of gearing properly for the designed load.

The loads on an FRC drivetrain can be to accelerate the robot, to maintain speed by overcoming drag, or to push with some force at a constant speed (perhaps close to zero).

In the third case, the load will be continuous, so you should definitely heed the current figures Ether's shown. You don't want to be pulling 80A per motor continuous, or you will pop some breakers (a 40A breaker in about a second, or for 6 six motors for a total of 480A, the main breaker in a few seconds as well).

In the second case, the load will be continuous as well, but much much lower.

In the first case, the acceleration is only transient. 6 CIMs geared 5:1 on 4" wheels get to 90% of top speed (about 18 ft/s with no loss) in about half a second. They are stalled briefly for the beginning of this, but then quickly approach free speed, so there aren't too many amps in too few seconds. In comparison, 6 CIMs geared 13:1 on 4" wheels get to half the high gear's top speed in about a quarter of a second, so while you are putting less load on the system (getting to 90% of low gear's top speed in just 0.1 seconds), the performance improvement due to starting in low gear and shifting at the right time is hardly noticable at all. Even at a distance of 5 ft, the low gear takes 50% longer to complete the traversal than the high gear, and of course it's even more in favor of high gear for longer distances. For this reason, we often remain in high gear unless we see that we're applying load continuously (pushing).

These numbers are all pretty approximate (and also coincidentally almost match our 2014 drive), but I think they show why high gear is important, too. Note that the ability for this high gear to still accelerate as it does is in part due to the 2 extra CIMs in the drive. For a 4 CIM drive, you would not get the same performance over short distances at these reductions, and low gear would be more favored.

[JoshD99]

I am the main driver for team 4327. The robot we built this year moves at about 16 FPS. At first, it was difficult to drive. But now, after becoming very familiar with the robot, I can drive it well, others on the team who don't usually drive, however, do have issues. So I don't think, with practice, a robot with a speed of 20 FPS would be an issue.